Update Github Actions workflow for Windows builds

MSYS2 has deprecated the MINGW64 environment which uses MSVCRT and this
project will no longer support that environment. Instead it will use
MSYS2's UCRT64 environment with its MinGW toolchain and CLANG64
environment with its Clang/LLVM toolchain, both using the UCRT runtime,
for Windows debug builds.

Author: leonlynch

.github/workflows/windows-build.yaml

@@ -17,8 +17,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { sys: mingw64, env: x86_64, build_type: "Debug", lib_type: "static", shared_libs: "NO", fetch_deps: YES }
-          - { sys: mingw64, env: x86_64, build_type: "Release", lib_type: "dll", shared_libs: "YES", fetch_deps: NO }
           - { sys: ucrt64, env: ucrt-x86_64, build_type: "Debug", lib_type: "static", shared_libs: "NO", fetch_deps: YES }
           - { sys: ucrt64, env: ucrt-x86_64, build_type: "Release", lib_type: "dll", shared_libs: "YES", fetch_deps: NO }
           - { sys: clang64, env: clang-x86_64, build_type: "Debug", lib_type: "dll", shared_libs: "YES", fetch_deps: NO }